Motor vehicles having a hybrid drive include, in addition to a conventional internal combustion engine, one or possibly more associated electric machines that are able to be coupled or fixedly connected to a drive shaft in the drive train of the motor vehicle and able to operate both in generator mode and in motor mode. When in generator mode, the electric machines are driven by the internal combustion engine and are able to generate electric current to supply loads of the motor vehicle, whereas in the motor mode, the electric machines are supplied with current from the vehicle battery to convert this current into motive power for the motor vehicle or into starting power for the internal combustion engine. In addition to the vehicle battery as a power storage device, a flywheel or a different kinetic energy storage device could be provided, for example, to allow kinetic energy released during braking to be stored and supplied to loads of the motor vehicle via the electric machine, or to be supplied to the drive train thereof at a later time.
During speed-controlled idle operation of the internal combustion engine, the electric machine is automatically operated in generator mode.
In motor vehicles having a hybrid drive, the open and closed-loop control tasks required for the operation of the internal combustion engine and the electric machine are generally distributed in the same way as in conventional motor vehicles. This means that the control of the rotational speed of the internal combustion engine is assumed by the electronic engine management system, which measures the speed and calculates control variables, such as injection quantity, ignition angle, or air quantity for the internal combustion engine, using suitable control methods so as to adjust or maintain the predetermined idling speed. In contrast, the voltage regulation of a vehicle electrical system and a charging control of a battery of the motor vehicle are accomplished in that power required by the vehicle electrical system or for charging the battery is provided by the electric machine with the aid of a controller of the electric machine.
However, the functions performed in the electronic engine management system to control the speed of the internal combustion engine are very complex because the control can only be carried out at the moment of ignition of the internal combustion engine, which results in a idle-speed control that is moderately fast and more or less smooth, depending on the number of cylinders. Moreover, a relatively high degree of complexity is necessary because the control must be stable under all operating conditions and protected against numerous disturbances.
In contrast, the speed of an electric machine can be controlled relatively easily and, moreover, torque control can be implemented with little effort and good results.
A motor vehicle having a hybrid drive and a method for idle-speed control of a hybrid drive of a motor vehicle are described in published German Patent Application DE 195 32 163. The method is used to reduce rotational irregularities of a shaft, in particular of the drive shaft of an internal combustion engine, and an electric machine coupled, or able to be coupled, to the drive shaft is controlled in such a manner that the rotational irregularities thereof are reduced. Moreover, due to its speed and performance, the controller responsible for controlling the electric machine may also be used to perform engine management tasks, including, for example, control of the speed of the internal combustion engine during idling, while on the other hand, the engine management system may assume one or more tasks of the controller of the electric machine, such as control torque of the electric machine.
In this connection, however, problems may arise with respect to the power demands of the electrical system of the motor vehicle because the vehicle electrical system must be subordinated to the requirements of the idle-speed control when the intention is to adjust and maintain a predetermined idling speed with the aid of the electric machine. If, for example, immediately after the start of the motor vehicle when the internal combustion engine is idling, an electrical load, such as a rear-window heater, is switched on while at the same time a battery charge controller of the motor vehicle requests current to be supplied for charging the vehicle battery, then the electric power required for this from the electric machine cannot be provided immediately because this would in turn have negative effects on the control of the speed of the internal combustion engine.